Method for manufacturing a core

ABSTRACT

The invention relates to a method for manufacturing a core ( 1 ) for producing a leading edge of a fan blade. The method comprises the steps of: (a) providing an initial core ( 11 ) made of a nickel-base alloy, (b) thermal spraying of a layer ( 12 ) of a cobalt-base alloy, comprising chromium and at least one element of tungsten and/or molybdenum, on the initial core.

FIELD OF THE INVENTION

The invention relates to the field of the manufacture of the leading edge for organic matrix composite (OMC) fan blades.

STATE OF THE ART

It is recalled that a blade conventionally has a leading edge and a trailing edge. The leading edge corresponds to the front part of the airfoil which faces the air flow and divides the air flow into a pressure flow and a suction flow, and a trailing edge which corresponds to the rear part of the airfoil. In the environment of a turbomachine, of an aircraft, it is possible that solid debris is caught in an air flow circulating in the turbomachine. This debris will then strike the leading edge of a blade and may damage it. Consequently, it is common practice to reinforce the leading edge of the blades. This technical provision is all the more important in the case of an OMC blade. Indeed, the composite material may not be able to resist perforation.

For example, it is known from the prior art to have an OMC fan blade with a titanium leading edge.

The document FR1051992 describes a known method for manufacturing such a blade. This method proceeds as follows: two formed sheets, pressure side and suction side, are shaped, via a hot isostatic pressing operation, around a refractory alloy core whose geometry corresponds to the internal geometry of the desired leading edge. After shaping, the core, which is a reusable tool, is removed and the leading edge is machined only on its outer surfaces to obtain the final geometry of the part.

This method makes it possible to control the internal shape of the leading edge cavity, which is a replica of the blade on which the leading edge will be placed, thus precluding re-machining of the leading edge internal cavity. In addition, this technique makes it possible to control and facilitate final machining operations thanks to the presence of the internal core, which both stiffens the assembly and provides integrated dimensional references, thus avoiding the need, as for other techniques, for complex machining tools. Thus, these different provisions induce a significant cost reduction of the manufacturing range because of the reuse of the cores, considered then as tools.

For this so-called core shaping technique, the core must have three main features related to the fact that the shaping step is carried out via a high-temperature thermomechanical cycle, of the order of 800-1000° C., during which the core is in contact with the titanium leading edge elements for several hours:

-   -   The core must be non-deformable in the thermomechanical range of         the leading edge manufacture to ensure the shape of the leading         edge internal cavity     -   The core must not allow any chemical reaction between its         material and the leading edge material     -   The core must not allow any adhesion or bonding between its         material and the leading edge material

The first feature, related to the choice of the core material, eliminates the need to machine the leading edge internal cavity.

The second avoids or minimizes chemical decontamination of the leading edge internal cavity surfaces.

The third condition is the reuse of the cores and therefore the economic viability of this technique.

The last two features are linked and require a special treatment of the core. Indeed, the metal alloys selected for the core are nickel- or cobalt-based alloys in order to be sufficiently rigid not to deform during the high-temperature shaping cycles. However, this type of alloys, when in contact at high temperature with the titanium alloys of the part, are reactive with each other and form solid solutions or intermetallic compounds, which leads at best to a contamination of the titanium alloy, and at worst to a totally unacceptable bonding between the nickel/cobalt and the titanium.

It is therefore essential to carry out a suitable treatment of the core to avoid contamination and bonding.

A technical solution consists in interposing an anti-diffusion barrier between the two metallic alloys in contact, i.e., the nickel- or cobalt-based alloy of the core and the titanium alloy of the leading edge, which will undergo a high-temperature thermomechanical treatment for several hours.

To this end, a method described by the document FR 1653221 is the nitriding or nitro-carburizing of the nickel- or cobalt-based core. This treatment generates a surface layer rich in nitrogen and carbon of a few tens of microns on the surface of the core ensuring the role of anti-diffusion barrier.

However, tests carried out on a scale of one using this technique have revealed deficiencies in terms of the effectiveness of the anti-diffusion barrier generated by the nitriding or carbo-nitriding of the nickel- or cobalt-based core. Traces of contamination are observed on the internal zones of the titanium leading edge and degradation of the nitrided layer of the core can be observed as early as the first pressing cycle, which greatly compromises the ability to reuse the core many times and therefore the economic model of the technique.

DISCLOSURE OF THE INVENTION

In this context, the objective of the present invention is to provide a process for manufacturing a core for the production of a leading edge of a fan blade, which meets the three criteria stated above: thermomechanical non-deformability, chemical neutrality with respect to the leading edge, and absence of adhesion to the leading edge.

According to a first aspect, the invention proposes a process for manufacturing a core for the production of a leading edge of a fan blade characterized in that it comprises the steps of:

-   -   (a) providing an initial nickel-based alloy core,     -   (b) thermally spraying a layer of a cobalt-based alloy,         comprising chromium and at least element among tungsten and/or         molybdenum, onto the initial core.

Particularly advantageously, the thermal spraying of this type of alloy, which is resistant to hot friction, provides a chemical inertia which makes it possible to create an anti-diffusion barrier between the nickel-based alloy core and the titanium sheets used for the manufacture of the fan blade. Thus, this process makes it possible to cover the core with a layer guaranteeing thermomechanical stability, protecting it from chemical contamination and avoiding possible adhesion of the blade parts to the core.

The cobalt-based alloy can comprise carbon.

The cobalt-based alloy can comprise between 26% and 32% molybdenum, between 7% and 20% chromium, between 1% and 5% silicon, and less than 1% carbon.

The cobalt-based alloy can comprise between 25% and 35% chromium, between 0% and 10% tungsten, between 0% and 7% molybdenum, between 0% and 4% nickel, between 1% and 3% silicon, and less than 2% carbon.

The cobalt-based alloy can comprise between 28% and 30% chromium, between 1.2% and 1.6% carbon, and between 1.2% and 1.8% silicon.

The layer can have a hardness comprised between 40 and 65 HRC.

The deposited layer can have a thickness comprised between 100 microns and 2 millimeters, preferably 500 microns.

Step (b) can be performed using a method selected from: supersonic flame spraying, blown arc plasma spraying, or plasma torch deposition.

The process can further include, following the thermal spraying step (b), a step (c) of heat treatment of the layer, carried out between 800 and 1000° C.

The process can further include, following the thermal spraying step (b), a grinding step (d).

The process can further include, following the thermal spraying step (b), a heat treatment step (e) in air, comprising a first stage of about thirty minutes at a temperature comprised between 300° C. and 400° C., a second stage of about thirty minutes at a temperature comprised between 500° C. and 700° C., a third stage of about thirty minutes at a temperature comprised between 800° C. and 1000° C., and a cooling in ambient air, to oxidize the layer.

According to a second aspect, the invention proposes a nickel-based alloy core obtained by a process according to the invention, said core having a layer (12) composed of a cobalt-based alloy comprising chromium and at least element among tungsten and/or molybdenum and/or carbon and/or silica obtained by thermal spraying of a cobalt-based alloy, comprising chromium and at least element among tungsten and/or molybdenum.

DESCRIPTION OF THE FIGURES

Further features, purposes and advantages of the invention will emerge from the following description, which is purely illustrative and non-limiting, and which should be read in conjunction with the appended drawings in which:

FIG. 1 is a microscopic cross-sectional view of the surface of a core according to the invention.

FIG. 2 is a block diagram of a process according to the invention.

Throughout the figures, similar elements have identical reference marks.

DETAILED DESCRIPTION OF THE INVENTION

Manufacturing Process

According to a first aspect, the invention relates to a process for manufacturing a core 1 for the production of a leading edge of a fan blade, the process essentially comprises the steps of:

-   -   (a) providing an initial core 11 of nickel-based alloy,     -   (b) thermal spraying a layer 12 of a cobalt-based alloy,         comprising chromium and at least element among tungsten and/or         molybdenum, onto the initial core 11.

The step (a) of providing the initial core can be performed by machining a nickel-based alloy block to obtain the initial core 11.

Next, the initial core 11 can be sandblasted (step (a1)) to increase the roughness of its outer surface. This arrangement advantageously facilitates the adhesion of the layer 12 sprayed in step (b).

Finally, the initial core 11 can be cleaned and degreased (step (a2)). This step ensures that the surface condition of the initial core 11 is optimal for the next projection step (b).

As mentioned above, the next step (b) consists in thermally spraying a layer 12 of a cobalt-based alloy, comprising chromium and at least element among tungsten and/or molybdenum, onto the initial core 11.

It is specified that tungsten, molybdenum and chromium can be present in the form of carbide.

Moreover, molybdenum can be used in an intermetallic mixture. It is recalled that an intermetallic mixture is a mixture comprising at least one metalloid. Metalloids are chemical elements whose properties are intermediate between those of metals and non-metals or are a combination of these properties. Metalloids are the following elements: boron, silicon, germanium, arsenic, antimony, tellurium and astatine. In this case, the metalloid preferentially used in combination with molybdenum is silicon.

According to a particular arrangement, the cobalt-based alloy may have the following composition, in percent by weight: between 26% and 32% molybdenum, between 7% and 20% chromium, between 0% and 10% tungsten, between 1% and 5% silicon, and less than 1% carbon.

Preferably, according to this arrangement, the cobalt-based alloy may comprise 28% chromium, 5.5% molybdenum, 2.5% nickel, 2% silicon, and 0.25% carbon.

Also preferably, according to the same technical arrangement, the cobalt-based alloy may comprise 29.5% chromium, 8% tungsten, 1.5% silicon, and 1.4% carbon.

According to another particular arrangement, the cobalt-based alloy may comprise between 26% and 32% molybdenum, between 7% and 20% chromium, between 1% and 5% silicon, and less than 1% carbon.

Preferably, according to this arrangement, the cobalt-based alloy may comprise 29% molybdenum, 8.5% chromium, 2.6% silicon, and less than 0.08% carbon.

Also preferably, according to this same particular arrangement, the cobalt-based alloy may comprise 28% molybdenum, 18% chromium, 3.4% silicon, and less than 0.08% carbon.

Thus, particularly advantageously, the thermal spraying of these types of alloys, which are resistant to hot friction, provides a chemical inertia which creates an anti-diffusion barrier between the initial core 11 made of nickel-based alloy and the titanium sheets used to manufacture the fan blade.

According to a first preferred technical arrangement, the thermal spraying can be carried out by supersonic flame, according to a method known as high-velocity oxy-fuel (HVOF) spraying or according to a method known as high-velocity air-fuel (HVAF) spraying. The HVOF method is particularly preferred because it consists of spraying the cobalt-based alloy at very high speed with a moderate temperature, which generates very little porosity in the deposited layer 12.

According to another technical arrangement, the layer can be sprayed by blown arc plasma. This method leads to a porous layer but with a good mechanical grip on the core surface. This method can optionally consider vacuum pumping during spraying.

According to another technical arrangement, the layer can be deposited by a plasma torch using the so-called plasma transferred arc (PTA) method. This method makes it possible to obtain a layer 12 that is thicker, more compact and metallurgically bonded to the substrate than with the previously described methods, which will then be reworked.

Following the deposition of the layer 12 by thermal spraying, the process may include a step (c) of heat treatment of the layer 12, carried out between 800° C. and 1000° C. Preferentially, step (c) is carried out between 850° C. and 900° C. This heat treatment is used to relax the internal stresses induced by the deposition of the layer 12, in the previous step.

Next, the process may include a grinding step (d) to reduce a thickness of the layer 12. As a result of this step, the layer 12 may have a thickness comprised between 100 and 500 microns. This step may also serve as a practical check of the adhesion of the deposit.

After the grinding step, the process may include a heat treatment step (e) in air, comprising a first stage of about thirty minutes at a temperature comprised between 300° C. and 400° C., a second stage of about thirty minutes at a temperature comprised between 500° C. and 700° C., a third stage of about thirty minutes at a temperature comprised between 800° C. and 1000° C., and cooling in ambient air.

Preferentially, step (e) may include a first thirty-minute stage at 350° C., then a second thirty-minute stage at 650° C., then a third thirty-minute stage at 900° C., followed by cooling in ambient air.

Step (e), known as passivation, is used to surface oxidize the layer 12, which reduces the risk of chemical interaction between the layer 12 and the material used to manufacture the blade (most often titanium).

As a result of this manufacturing process, the layer 12 can have a hardness comprised between 35 and 65 HRC, preferably between 45 and 55 HRC. It is specified that the hardness is expressed and measured according to the so-called Rockwell test, using an indenter to which an initial load is applied and then an additional load. The hardness is measured by comparing the indentation depth of the indenter when the initial load is applied and when the additional load is applied. For the HRC scale, in this case, the test is performed with an indenter consisting of a diamond cone of circular section with a spherical rounded tip of 0.2 millimeters. Moreover, the initial load applied is 98 N and the total load (corresponding to the initial load plus the additional load) is 1471.5 N. One unit of HRC hardness corresponds to a penetration of 0.002 millimeters.

For thicknesses less than 400 microns, the HR15N scale is preferred because the initial load applied is only 29 N and the total load (corresponding to the initial load plus the additional load) is only 147.1 N.

It is also possible to use the Vickers (HV) method. The test is then performed with an indenter consisting of a diamond pyramid with a square base. The hardness is determined by measuring the two diagonals of the indentation. The load is adapted to the thickness of the layer: 5 to 10 kilograms for thicknesses ≤400 microns and 20 to 30 kilograms maximum for thicknesses greater than 400 microns.

In the last two cases (HR15N and HV) the HRC value is deduced from the conversion tables expressed in the current ISO and ASTM standards.

Thus, this process makes it possible to cover the initial core 11 with a layer 12 guaranteeing thermomechanical stability, protecting against chemical contamination, and avoiding possible adhesion of the blade parts to the core.

Core

According to a second aspect, the invention relates to a nickel-based alloy core 1 obtained by a process according to the invention. The core 1 has a layer 12 composed of a cobalt-based alloy comprising chromium and at least element among tungsten and/or molybdenum obtained by thermal spraying of a cobalt-based alloy, comprising chromium and at least element among tungsten and/or molybdenum. 

1. A process for manufacturing a core for the production of a leading edge of a fan blade, characterized in that it comprises the steps of: (a) providing an initial core of a nickel-based alloy, (b) thermally spraying a layer of a cobalt-based alloy, comprising carbon, chromium and at least element among tungsten and/or molybdenum, onto the initial core.
 2. The process as claimed in claim 1, wherein the cobalt-based alloy comprises between 26% and 32% molybdenum, between 7% and 20% chromium, between 1% and 5% silicon, and less than 1% carbon.
 3. The process as claimed in claim 1, wherein the cobalt-based alloy comprises between 25% and 35% chromium, between 5% and 10% tungsten, between 0% and 7% molybdenum, between 0% and 4% nickel, between 1% and 3% silicon, and less than 2% carbon.
 4. The process as claimed in claim 3, wherein the cobalt-based alloy comprises between 28% and 30% chromium, between 1.2% and 1.6% carbon, and between 1.2% and 1.8% silicon.
 5. The process as claimed in claim 1, wherein the layer has a hardness comprised between 40 and 65 HRC.
 6. The process as claimed in claim 1, wherein the deposited layer has a thickness comprised between 100 microns and 2 millimeters.
 7. The process as claimed in claim 1, wherein step (b) is performed using a method selected from: supersonic flame spraying, blown arc plasma spraying or plasma torch deposition.
 8. The process as claimed in claim 1 further comprising, following the thermal spraying step (b), a step (c) of heat treatment of the layer, carried out between 800 and 1000° C.
 9. The process as claimed in claim 1 further comprising, following the thermal spraying step (b), a grinding step (d).
 10. The process as claimed in claim 1, comprising, following the thermal spraying step (b), a heat treatment step (e) in air, comprising a first stage of about thirty minutes at a temperature comprised between 300° C. and 400° C., a second stage of about thirty minutes at a temperature comprised between 500° C. and 700° C., a third stage of about thirty minutes at a temperature comprised between 800° C. and 1000° C., and cooling in ambient air, to oxidize the layer.
 11. A nickel-based alloy core obtained by a process as claimed in claim 1, said core having a layer composed of a cobalt-based alloy comprising chromium, carbon, and at least element among tungsten and/or molybdenum and/or carbon and/or silica obtained by thermal spraying of a cobalt-based alloy, comprising chromium and at least element among tungsten and/or molybdenum. 